Use WritableDir in execute_cmdstager for exploit/linux/http/f5_bigip_tmui_rce

[wvu]

Since we're using WritableDir in script_path for upload_script, we can also use it in :temp for execute_cmdstager.

msf6 exploit(linux/http/f5_bigip_tmui_rce) > grep WritableDir advanced
   WritableDir             /tmp                                                yes       Writable directory
msf6 exploit(linux/http/f5_bigip_tmui_rce) >

https://github.com/rapid7/rex-exploitation/blob/9c19df14e66d381e318996478e5d55018a6061c6/lib/rex/exploitation/cmdstager/base.rb#L26-L31

Fixes #13807. Bughancement.

[wvu]

You forgot about #11467, didn't you?